To answer my own question. Say I want to keep track when a certain record is printed or not:

You can define a new function in the model's .py file:

def set_record_printed(self):
    self.ensure_one()
    self.printed = True
    self.timestamp_printed = fields.Datetime.now()

Where self will be the record on which you want to execute the function.

Then in the report QWeb template just call that function on each record.

<t t-esc="product.set_record_printed()"/>